I had same issue and raised a ticket, it’s a known bug. Here’s their response…

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Our team is aware of this issue with Alpine skiing activities synced from Apple Health and it is due to limited distance data that is being recorded in the files that are synced from Apple Health. While GPS and other sensor data (like heart rate) are being recorded on a 1-2 second basis we have recently noticed that Apple Health has started to include distance data points in their files but are only recording them every few minutes which is resulting in large jumps of distance which is impacting the Strava side calculations with speed and run counts but can also impact other stats as well.
 
Our engineers are working on a way to better handle files like this with limited distance data points and have also reported the issue to our contacts at Apple, but we do not have an eta for any fixes due to the nature of the features involved in this process.
 
In the meantime, we recommend that you consider recording any alpine skiing activities on the Strava Apple Watch app, and our Apple Health integration is able to send that information to their side so you will still get credit for those activities. You can also correct the distance on your activities that were already uploaded to Strava to use distance that is only calculated using recorded GPS points which are recording at a normal cadence, unlike the recorded distance. 

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thanks for the heads up!